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WARNING: Some people try to build this with an optocoupler with zerocrossing coz 'that is better' right? Some are even told in electronics shops it is better to use such an optocoupler. WRONG. This will only work with a random fire optocoupler: NOT igniting at zerocrossing is the principle of this dimmer.

Switching an AC load with an Arduino is rather simpel: either a mechanical relay or a solid state relay with an optically isolated Triac. (I say Arduino, but if you use an 8051 or PIC16F877A microcontroller, there is stuff for you too here.)

It becomes a bit more tricky if one wants to dim a mains AC lamp with an arduino: just limiting the current through e.g. a transistor is not really possible due to the large power the transistor then will need to dissipate, resulting in much heat and it is also not efficient from an energy use point of view.

Phase cutting
One way of doing it is through phase control with a Triac: the Triac then is fully opened, but only during a part of the sinus AC wave. This is called leading edge cutting.
One could let an Arduino just open the Triac for a number of microseconds, but that has the problem that it is unpredictable during what part of the sinus wave the triac opens and therefore the dimming level is unpredictable. One needs a reference point in the sinus wave.
For that a zero crossing detector is necessary. This is a circuit that tells the Arduino (or another micro controller) when the sinus-wave goes through zero and therefore gives a defined point on that sinus wave.
Opening the Triac after a number of microseconds delay starting from the zero crossing therefore gives a predictable level of dimming.

Pulse Skip Modulation
Another way of doing this is by Pulse Skip Modulation. With PSM, one or more full cycles (sinuswaves) are transferred to the load and then one or more cycles are not. Though effective, it is not a good way to dim lights as there is a chance for flickering. Though it might be tempting, in PSM one should always allow a full sinuswave to be passed to the load, not a half sinus as in that case the load will be fed factually from DC which is not a good thing for most AC loads. The difference between leading edge cutting and PSM is mainly in the software: in both cases one will need a circuit that detects the zero crossing and that can control a triac.

A circuit that can do this is easy to build: The zero crossing is directly derived from the rectified mains AC lines – via an optocoupler of course- and gives a signal every time the wave goes through zero. Because the sine wave first goes through double phased rectification, the zero-crossing signal is given regardless whether the sinus wave goes up through zero or down through zero. This signal then can be used to trigger an interrupt in the Arduino.

PWM dimming
PWM dimming, as in LEDs is not done frequently with AC loads for a number of reasons. It is possible though. Check this instructable to see how.

It goes without saying that there needs to be a galvanic separation between the Arduino side of things and anything connected to the mains. For those who do not understand 'galvanic separation' it means 'no metal connections' thus ---> opto-couplers. BUT, if you do not understand 'galvanic separation', maybe you should not build this.

The circuit pictured here does just that. The mains 220Volt voltage is led through two 30k resistors to a bridge rectifier that gives a double phased rectified signal to a 4N25 opto-coupler. The LED in this opto-coupler thus goes low with a frequency of 100Hz and the signal on the collector is going high with a frequency of 100Hz, in line with the sinusoid wave on the mains net. The signal of the 4N25 is fed to an interrupt pin in the Arduino (or other microprocessor). The interrupt routine feeds a signal of a specific length to one of the I/O pins. The I/O pin signal goes back to our circuit and opens the LED and a MOC3021, that triggers the Opto-Thyristor briefly. The LED in series with the MOC3021 indicates if there is any current going through the MOC3021. Mind you though that in dimming operation that light will not be very visible because it is very short lasting. Should you chose to use the triac switch for continuous use, the LED will light up clearly.

Mind you that only regular incandescent lamps are truly suitable for dimming. It will work with a halogen lamp as well, but it will shorten the life span of the halogen lamp. It will not work with any cfl lamps, unless they are specifically stated to be suited for a dimmer. The same goes for LED lamps

If you are interested in an AC dimmer such as this but you do not want to try building it yourself, there is a somewhat similar dimmer available at www.inmojo.com, however, that is a 110 Volt 60Hz version (but adaptable for 220 50Hz), that has been out of stock for a while. You will also find a schedule here.

NOTE! It is possible that depending on the LED that is used, the steering signal just does not cut it and you may end up with a lamp that just flickers rather than being smoothly regulated. Replacing the LED with a wire bridge will cure that. The LED is not really necessary. increase the 220 ohm resistor to 470 then


STOP: This circuit is attached to a 110-220 Voltage. Do not build this if you are not confident about what you are doing. Unplug it before coming even close to the PCB. The cooling plate of the Triac is attached to the mains. Do not touch it while in operation. Put it in a proper enclosure/container.

WAIT: Let me just add a stronger warning here: This circuit is safe if it is built and implemented only by people who know what they are doing. If you have no clue or if you are doubting about what you do, chances are you are going to be DEAD!
DO NOT TOUCH WHEN IT IS CONNECTED TO THE GRID

Materials
Zerocrossing
4N25 €0.25 or H11AA1 or IL250, IL251, IL252, LTV814 (see text in the next step)
Resistor 10k €0.10
bridge rectifier 400 Volt €0.30
2x 30 k resistor 1/2 Watt (resistors will probably dissipate 400mW max each €0.30
1 connector €0.20
5.1 Volt zenerdiode (optional)

Lamp driver
LED (Note: you can replace the LED with a wire bridge as the LED may sometimes cause the lamp to flicker rather than to regulate smoothly)
MOC3021 If you chose another type, make sure it has NO zero-crossing detection, I can't stress this enough DO NOT use e.g. a MOC3042
Resistor 220 Ohm €0.10 (I actually used a 330 Ohm and that worked fine)
Resistor 470 Ohm-1k (I ended up using a 560 Ohm and that worked well)
TRIAC TIC206 €1.20 or BR136 €0.50
1 connector €0.20

Other
Piece of PCB 6x3cm
electric wiring

That is about €3 in parts

Step 1: Arduino controlled light dimmer: The PCB

You will find two pictures for the PCB: my first one, that I leave here for documentation purposes and a slightly altered new one. The difference is that I left out the zenerdiode as it is not really necessary and I gave the LED itś own (1k) resistor: it is no longer in series with the Optocoupler, that now has a 470 Ohm resistor. I made the PCB via direct toner transfer and then etched it in a hydrochloric acid/Hydrogenperoxide bath. There are plenty of instructables telling how to do that. You can use the attached print design to do the same. Populating the print is quite straightforward. I used IC feet for the opto-couplers and the bridge rectifier.
Download the print here.
Note: You need Fritzing for this. For the direct toner transfer, the printed side of the printed pdf file, goes directly against the copper layer for transfer. Once it is transferred, you will be looking at the ink from the other side and thus see the text normal again. I made slight alterations in thePCB: I removed the zenerdiode and the LED is no longer in series with the optocoupler.

I used a TIC206. That can deliver 4 amperes. Keep in mind though that the copper tracks of the PCB will not be able to withstand 4 Amperes. For any serious load, solder a piece of copper installation wire on the tracks leading from the TRIAC to the connectors and on the track between the two connectors.

In case it is not clear what the inputs are: from top to bottom on the second picture:
+5Volts
Interrupt signal (going to D2 on arduino)
Triac signal (coming from D3 on Arduino)
Ground

NOTE:
If you have an H11AA1or IL 250, 251 or 252 opto-coupler then you do not need the bridge rectifier. These have two anti-parellel diodes and thus can handle AC. It is pin compatible with the 4N25, just pop it in and solder 2 wire-bridges between R5 and + and R7 and -. The LTV814 is not pincompatible

<p>Hello,</p><p>I want to dim a 12v halogen lamp. I have a dimmable halogen psu, but I have to drive it with a TRIAC, from what I understood.</p><p>Is this circuit suitable for my needs?</p>
<p>You mean that it is a 12 Volt halogen lamp that is attached to a PSU that goes into the grid? and that PSU is labeleld as 'dimmable'.<br>If it is for a traditional TRIAC dimmer then you will have a good chance that this circuit will do the job </p>
<p>thanks for your reply,</p><p>I tried it with my configuration, but seems that my electronic psu prefers trailing-edge dimming. I get a strange behaviour of the halogen lamp while a normal lamp works great when attached directly to your circuit.</p>
<p>I have try the code for &quot;AC Voltage dimmer with Zero cross detection&quot; , I only can see the light glow and dim again and repeat again. Is this the result ? </p><p>for the code by using push button, I only see the light is flickering.</p>
<p>then most likely a hardware problem. Please check and recheck all yr connections</p>
<p>can I use just one &quot;bridge + 4N25 circuit part&quot; to get the zero cross for supply the arduino, and then use separate MOC3021+TIC206 circuit part for dimming each light? or its necesary the whole circuit for each light?</p>
<p>you only need one circuit for the zerocross interrupt.</p><p>if you want more lights you only need to copy the MOC3021+Triac part.<br>have a look at my <a href="http://www.instructables.com/id/3-channel-Dimmerfader-for-Arduino-or-other-microco/">3 lamp dimmer</a>.</p>
<p>thanks for reply, just 2 more questions, this circuit is better (dutty cycle/cost) than mosfet one you optimize, for a xmas light show? (I planned to run several hours)<br>wich kind of lights can be used? normal classic ones? rice type? led type?</p><p>note: I'll not put more than 2 strips together for each TIC206 part</p>
<p>I think that if you have those normal classic xmas lights in series on a string, then it doesnt make much difference which one you are using. As long as you keep the TIC cool it can take quite a load. If it are lights that work with a transformer than the PWM circuit is not suitable.<br>With a transformer the TRIAC circuit is not ideal but it will most likely do the job.<br>Merry xmas</p>
<p>I made it. </p><p>https://youtu.be/N_Zx1xARwtM</p>
<p>looks good sureshmali. Obviously you added a bit too it :-)</p>
<p>here is my schematic, after i double check the coding and circuit, the lamp still glow at 100%, i dont know what is happening here, my component is 4n26, moc3021, Triac bt136 and bridge rectifier. You know what is happening here? Please help me</p>
<p>circuit looks OK. I am not sure if I asked before but did you try the original dimmer demo from the instructable? did that work properly? That would be very helpful to see if the problem is in yr software or your hardware</p>
<p>I see , but I have try the original dimmer code already , I just saw the light flickering .</p>
<p>well that strongly suggest indeed that the problem is in your hardware. I cannot see anything wrong with yr circuit. When you tried the original program, did you include the LCD that I see in your schedule? Or was it just the program without any additions?<br> Could you recheck all connections and soldering joints?</p>
<p>Thanks for sharing! I have two questions related to your schematics.</p><p>Can I simply omit the bridge rectifier and still use a 4N25? The internal diode would block negative signal, and I would think software can handle it (especially as I plan to do only bursts of full-waves, not chopping them at a higher frequency).</p><p>Also why do you use two 30K resistors where only one 60K would seem to be enough? Thanks again :)</p>
<p>you would only have one phase and therefore you wouldnt have a zerocross, just a signal that lingers at zero for half of the cycle time.<br>You can use 1 60k but it would need to be of the double wattage that the 30k resistors are. That is one reason, The other is that by using 2 resistors the full 220 signal would be kept out of the PCB as much as possible<br></p>
Got it, thanks for your reply :)<br>The raising edge would trigger an interrupt at the beginning of each cycle then (and it is no more a zerocross indeed!).<br>
<p>Well ofcourse the signal doesnt need to go through zero ofcourse, just be low enough to open the 4n25 and if you trigger on the rising or falling flank you would still get an interrupt, but you would only get an interrupt for half of the cycle</p>
<p>What about false zero cross triggering due to other Triacs firing at different phases? </p>
<p>I do not understand what you are exactly asking</p>
<p>When using several channels of the same circuit to dim several loads and each load is at different value.</p><p>In this case when you fire the Triac it will introduce some noise on the mains.</p><p>This noise may be false detected as a Zero cross by the zero cross circuit, and case flicker .</p><p>Did you face such problem before?</p>
<p>no I didnt have any of those problems. It is important though to have a clean PSU as the grid is full of noise whether Triac induced or not.<br>Anyway, I never had any problem</p>
<p>Can i have a code for up down button control light dimmer ?</p>
<p>i have sent you already when i first received your email. check yr spam folder maybe. but will also send again</p>
<p>I receive it already..thanks :)</p>
<p>turns out i sent it to you on the 14th. 5 days ago. I just resent it</p>
<p>This is my email : joshuasamuel@klt.edu.my</p><p>I see, this is the problem i need to fix it, my final year project is to use the button to dim the light bulb, and i also use the LCD display to display the dim level. But after i power on the supply, the bulb only glow 100%, the circuit connection is right, so i guess is the code problem (T.T) . I hope you can help me.</p><p>I still waiting for you to send me the original code. Thanks :)</p>
<p>Hello! Im a student and I have basic knowledge of electronics and the arduino. I was thinking if it would be possible if I add sensors while using this circuit for dimming. Particularly, pir sensor and photodiode. Moreover, i wanted to also add bluetooth module for additional manual control of the led bulb. Is it possible or is it too much for the arduino to handle? I have lots or questions for this project because its very nice and i want to add more features. thank you. kudos also to your very detailed instructable</p>
<p>it is not too much to handle for the arduino, but just know that the first program is a demo program that spends most of its time in a delay loop. If you want to do something else you need to replace he delays with some sort of timer function. Also in writing yr program keep in mind that there is an interrupt 100 times a second</p>
<p>after i used the code for up down button, the light is constantly on 100%, what is happening here ?</p>
<p>johsim in the time I wrote this instructable I thought it would be beneficial to people if I added some codes besides the basic program. But it is a long time since I tested that specific program as I didnt want to use any buttons. Now it is very well possible that in putting the code on this webpage something went wrong. I took a quick look and it seemed OK, but If you p, me your email address (or leave it in the comments if ok with you. I will send you the original code. That however also has an LCD section in it but you can just ignore that.</p>
<p>This is my email : joshuasamuel@klt.edu.my</p><p>I see, this is the problem i need to fix it, my final year project is to use the button to dim the light bulb, and i also use the LCD display to display the dim level. But after i power on the supply, the bulb only glow 100%, the circuit connection is right, so i guess is the code problem (T.T) . I hope you can help me. Thanks </p>
<p>Hi to everyone,</p><p>I built this circuit because I was interested like many in controlling a 230 V AC Motor Fan.<br>First built for simple loads like a light bulb worked well and than for complex loads (inductive) like a motor. So I used the circuit with the snubber. Works well for i think 10 minutes than it starts to burn ;). Better said the 180 Ohm Resistor in the snubber circuit is to small and it will catch fire. I haven't done any calculations for the snubber yet but i will keep it up to date and try a different resistor. <br>Just wanted to warn people and I really recommend a heatsink for the Triac. <br>And I used an analogPin instead of the digital interrupt (got problems with it do not know why, but works well with analog Pin).</p><p><br>So I will make an update in the next few weeks for the snubber.</p><p>Anyway great circuit :)</p>
<p>thanks<br>Depending on the load yes a heatsink is advisable.<br>Snubber circuits indeed need to be able to deal with the spikes going through them.<br>I am surprised you had to resort to the analog pin, you must have changed the program then as there is no interrupt on the analog pin, but ofcourse polling is always possible</p>
<p>What is the minimum load on this dimmer ?<br>I have a parallel set of 4 dimmable led bulbs (5.5 Watts max each, 230V AC), will that work ?</p>
<p>the minimum load I tried was 40 watt and that was without problem. whether you can dim LED lamps largely depends on the LED lamp. Many dimmable lamps need dedicated dimmers, but if these are LED lamps that are for what is called a 'traditional' dimmer you have a good chance of it working</p>
<p>I made this for five bulbs . but there is a flickering. flicker rate is not constant, flicker rate change with the time and its looks periodic. its increase with time and then decrease. then flickering stop, after about 1 second its start flicker again. can you please tell me the problem. its not the number of bulbs i have used, even with one bulb there is flickering . i changed the resister values from 33k to 16.5k, detect zero cross using a 12v transformer, replace optocouplers. changed the 4n25 pull up resister from 10k to another value. nothing helps. flickering rate and pattern remaining same . can you please help me. thank you for this great instructable .</p>
<p><a href="http://www.instructables.com/member/%E0%B6%85%E0%B6%BA%E0%B6%B1%E0%B7%8A%E0%B6%B8">අයන්ම</a> I am not sure if you saw my 3 channel light dimmer, but that is just for your information.<br>There can be various reasons for flickering: software and hardware, so lets see where the problem is. I presume you have juist built the triac circuit 5 times and the zerocross one time.<br>What happens if you ignore the 4 channels and just use one and use my demo program? What happens then?</p><p>Flickering coz of hardware is usually because of a dirty psu. What happens if you feed your microcontroller from a battery?</p><p>I presume you used the 16.5k to get a smaller pulse, but apparently that was not the solution. Though you can use a transformer to get the pulse off but the quality of the transformer is of importance.</p><p>Do you have flickering over the entire range? Is there also flickering if you write one value to the optocoupler? say, 70.</p><p>Disconnect your microcontroller and put 5 volt on the entrance of your circuit. Do you have flickering too? </p><p>Anyway.. try that and try also feeding the microcontroller with a battery </p>
&quot;Disconnect your microcontroller and put 5 volt on the entrance of your circuit. Do you have flickering too?&quot; I didnt understand this. What do you mean by entrance
<p>the entrance is where you would normally put the output of the arduino</p>
<p>i tested it with your sample code ,which does not use the timer interrupts. it worked fine without flicker. flicker is only visible with the timer interrupt code. please help me to solve this problem. thank you. </p>
<p>Are you using an Arduino? which one?</p>
Yes. An Uno
<p>The Timer program as you can see in the legenda is not made by me, but it sounds as if somehow your timing is abit off.<br>Now there are two timer programs I think, in which step is the one you are using?<br>I am just going to make sure the timer is working correct</p>
Yes.i think the problem lies with intenal and external inturrupts. When we use both in same sketch it doesnt work well. Thats may be the reason when we use delay for dimming,the flickering doesnt apper. I have increased the number of brighness steps for 320. I called the ISR with 30microseconds delays and the flickering reduced to almot zero. To reduce the time concumed for ISR i used Direct port manipulation insted of digitalWrite and pinMode.
<p>ok so you did change rhe sketch, then not much use i check the one here. Direct port manipuation is good, makes it less portable but not really a problem for dedicated project. Make sure you dont make the interrupt routines too long</p>
I sloved the problem completely. Thanks for the help.
<p>great. good to hear</p>
Mr. Bloke. Did you run the circuit with the timer inturrupt code. And did it work smoothly? .

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Bio: I am a physician by trade. After a career in the pharmeceutical world I decided to take it a bit slower and do things I ... More »
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